I was recently contacted by a '65 L-76 owner who felt his 225 RWHP @5500 was low, and I agreed. His engine is configured very similar to Mark Johnson's engine, which measured 278@6500, so 225 represents a nearly 20 percent reduction.

A series of compression and leakdown tests indicated more variation than one would expect for a relatively fresh engine, and one cylinder definitely appears to be a little weak, but not enough to drop peak power 20 percent. Peak torque of 270 was good - a little better than Mark's 263, but Mark's fan was probably consuming 15 lb-ft, and the other owner reported that the fan clutch probably did not tighten during the tests.

There was one major "external" difference. The car in question has OE sidepipes and Mark's car has the under-the-car system.

So I decided to see if I could come up with a flow model for the sidepipe system and analyze what effect it has on backpressure and power output.

The Engine Analyzer simulation program allows the user to model exhaust systems by stating flow rate at 1.5" Hg (20.4" H2O) depression, which is the same depression used to rate four barrel carburetors. Using some engineering intuition I came up with a model of 250 CFM per side, 500 total, and this jibes with test data. For example, Mark's 278 RWHP converts to 327 SAE net at the flywheel using 15 percent driveline loss, and EA's prediction is 324 so they correlate within one percent.

Using some good verbal descriptions of the OE sidepipe system I modeled it as a 1 3/4" pipe, which is probably giving it the benefit of the doubt, and the flow ratio at constant depression is proportional to the area ratios, which is a function of the square of the diameters ratio, which is 0.49.

In other words, one side of the under-the-car system flows as much as two sidepipes. Imagine a SHP engine with a single exhaust system!

Plugging 245 CFM total exhaust flow into EA increases exhaust backpressue on Mark's engine at 7000 revs from 3.3 to 9.5 psi - nearly triple- dropped peak power ten percent, and knocked 500 revs off the useable power bandwidth, reducing the upper boundary from 7000-7200 to no more than 6500.

It's still not enough to account for 20 percent loss, but I'd be willing to believe that the sidepipes account for 15 percent loss and the slightly weak cylinder 5 percent, and this is supported by the peak torque data.

It's always been "heard on the street" that sidepipes are more restrictive than the under-the-car-system, but until now I had no quantification. Now I know, or at least have a pretty good idea.

If you want sidepipes and high peak power - forget it, they are mutually exclusive. I always felt that the under-the-car system was good. It may not be a wonder to behold visually, but it was very well engineered and works very well. Limiting backpressure to only about 3 psi with modest sound level is indicative of good engineering. Large free flowing manifolds, generous pipe sizing, and mufflers located as far back as possible is a textbook description of a good exhaust system. Now if someone could only do an accurate reproduction of the OE off-road mufflers, so we could have that great sound...


Though the under-the-car-exhaust is very efficient on a SHP small block, it's going to be restrictive on a SHP big block generating about 5.5 psi. Backpressure increases with the square of exhaust mass flow, so the sidepipe model yields about 16 psi backpressure on a SHP big block, which places exhaust backpressure in the range of seventies vintage Corvettes with the single bead bed converter, and your big block may be lunch for Mark's small block.

The other bit of insight is that if your exhaust system flows 80 percent or better (at 1.5" Hg. depression) of what a properly sized carburetor flows, then you've got a good system. So 500 CFM is good for a SB, but a high revving BB needs 700 CFM - 3" pipes, bigger mufflers... sorry, there's no room! My model for the C6/Z06 is 750 CFM.

It would not be that hard to test a complete exhaust system - one side from head pipe to tail pipe - on any flow bench that is suitable for testing cylinder head flow, but until customers start demanding real engineering data, they'll continue to get sold a bill of goods based on "look", "sound", and all sorts of other marketing foo-foo.

Duke

 

Lots of work Duke and great info, thanks. I'm still not taking my side pipes off though... . They are the 2.5" (2.25" thru the middle) sweet thunders though. No appreciable loss in power....the trade off? NOISE, and lots of it. Besides, I love setting off car alarms.

 

I agree completely, while I have not done a dyno session, my car woke right up with the bigger side pipes, I have had to rejet the carb and start all over tuning since retiring the OEM sidepipes.........

 

I certainly cannot speak for sidepipes vs undercar, but I can speak from personal experience regarding a 2in vs 2 1/2in undercar system. Both behind 2 1/2in manifolds and both on the same car and same engine. All that was changed was the pipes/mufflers between the manifolds and the tail pipe exit. I had 2in stainless on the 56 for many years, first behind a healthy FI 350 and then behind a SB400 with the same FI unit. In my heart I always knew the 2in stainless was restrictive, but just could not bring myself to remove and discard the stainless system. Finally, a couple of years ago I removed the stainless 2in and had my favorite muffler shop fabricate a beautiful, COMPLETE 2 1/2in system. The difference was extremely noticeable!

 

Is the sidepipe restriction the 90 deg. (?) bends ?

 

no, it's the tight crimping and packing inside the muffler



Sweet Thunder makes the REALLY LOUD and non-restrictive mufflers

the GNM version core is over 2 times the area of the stock design

 

Maybe you folks can help me.
My last 67 had a 327 and side pipes. I used 2 1/2 inch manifolds and 2 1/2 inch sdiepipes.
It was very loud.

My new 67 is a 327/350 and has the stock manifolds which I think are 2inch. It has what appears to be the orginal set of pipes which sound geat but need to be replaced.

My wife asked that we put the same pipes on because it sounds better. Are there pipes out there that sound like the orginal, but are not restrictive?

VG

 

 

Wow!
You learn something new everyday.
That bit of info is going to make it hard for me to sell my BB side exhaust.

 

I wouldn't argue with Duke's tech data........ however, I'd categorize it as "Factory" Side Pipes Are Power Killers (especially on BB cars)!

That said, I personally wouldn't have a midyear without them ..... (with big cores, or spiral baffles though).

You can put together a very clean flowing side pipe system with a little effort ... that is, if your not offended by the sound of a high performance car.

 

A ninety degree bend that close to the heads is incredibly restrictive. Probably halves the effective diameter of the pipe.

 

With this info in mind can someone explain why so many old Corvettes that I see vintage raced use sidepipes? Is it because there are no mufflers involved, the pipe diameters are much greater, they are not bent as severely, all of the above, or none of the above?

 

You know, I just may give that a go this winter (give me something to do while the snow is flying here in MI). I have a set of 2" Stahl's, and am in the process of process of reworking my "under cover" spiral baffle setup.... i have access to the flow measuring equipment........and i have a set of factory sidepipes....hum)

 

Well,..... if you have no mufflers, I'd think the side pipe setup would have less restriction, overall, than the under car system, plus cooler in the cockpit as well I'd guess, and probably lighter weight also

 

Look at the size of the sidepipe header tubes being used (and turned) 1 7/8 on smallblocks and 2 1/8 on rats. Compare that to a single 2 1/2 inch pipe with a 90 degree bend.

 

Probably all of the above.

I too believe a C2 must have side pipes to be real. If it's not making some noise and setting off car alarms, what's the fun? I'll drop a few ponies for the thrill of it!!

 

I replaced my 2 in under with 2.5 sides in late July. Car sounds and looks great, is much cooler inside driving on a hot day (no air) and the engine runs cooler by 15 degrees. If it doesn't go faster, it sure as heck sounds faster . Besides I'm way past racing age. I'll take cool any day.

 

I'm not sure if I should be worried or not....I have the Engine Analyser program too and have been modeling my engine for a few days, and have a pretty close model compared to the actual dyno numbers. Of course if I add a full exhuast system my power goes to crap. I have an undercar chambered exhaust, it is 2.5 inch expanding to around 3inch at the chambers but I think remains 2.5 inside diameter....no packing I can see, the chambers I think were made by Walker, but I am not sure now. I wonder how I can find out what these flow...my models say I will need about 750 cfm with 1000+ being even better.....I doubt they do the high end but it would be nice to get the 750. One question I have is as the pipes exit the car they neck down to 2.25....I can easily keep it at 2.5 if I want but I allowed the neck down on the idea that by the time the exhaust reached the back of the car it has cooled and slowed enough that the .25 reduction would give a little speed bump on exit helping the exhaust to get out away from the car and maybe even generate some extra flow....

Duke can you speculate on this exhaust? Without testing I am sure it is impossible to say anything for sure but just general ideas and would a cross pipe (I have no crossover in the intake) and or removing the final diameter reduction help my flow numbers?

Thanks
Darren

 

Professor Duke,

Thanks for taking the time to first do the research and second for sharing. I'll just sit back and try to take it all in.

Bud.

 

My analysis was for the OE optional side pipe system. The cars you see at vintage races have 3-4" open collector pipes running the length of the rockers - totally different system, When combined with a set of high flow efficiency 1 3/4" by 32" headers - the kind of headers made by the likes of Jere Stahl, not something from Jegs - and installed the system on Mark's engine it would add about 15 percent more power, even with the restrictive inlet manifold, but he probably wouldn't get very far before he got his first ticket for excessive noise.

Replace the SHP manifold and Holley with the single plane manifold architecture FI system and a cam with more overlap like the 30-30 and power would increase another 20 percent to about 430 net - same internals except the cam. Add more compression and some other tweaks and 450 net is in sight, but, of course, the bottom end of the torque bandwidths will move up the rev scale.

Once you eliminate the requirement for mufflers you open a whole other world of tuning opportunities, but conversely, a "racing cam" even with even modest backpressure will result in a poor road engine - poor torque bandwidth and only so-so top end power.

Road engines and racing engines are completely different animals.

Duke